The invention relates to a control circuit for thermal printheads having individual heating elements which can be selectively energized to raise the temperature of thermally sensitive paper in contact with the printhead so as to form a desired array of dots on the paper. In particular, the invention relates to a thermal printhead circuit constructed to facilitate multiplexing of the heating elements so as to limit the number of leads required to connect the heating elements to a power source.
In constructing a thermal printhead having many heating elements significant problems exist due to the large number of wires required to connect the individual heating elements to means used to selectively energize the elements. It is thus desirable to provide a construction which reduces the number of lead connections required while still permitting maximum flexibility in selectively energizing the heating elements. A matrix of heating elements can be employed in a thermal printhead in which the elements are connected in such fashion as to form a plurality of possible parallel circuits which results in a reduction in the number of lead connections required. A problem encountered with such a matrix is that when selected heating elements are energized, some current travels through the other elements via the plurality of parllel sneak circuits. Consequently, one or more of the unselected elements may dissipate enough energy to mark the thermally sensitive paper thus creating undesirable shadows.
A common solution to this problem is to employ a multiplexing technique using diodes to prevent unselected elements from being energized while other elements are energized for printing. This results in high fabrication costs due to the labor required to mount the diodes. Further, the space taken up by the diodes undesirably increases the physical dimensions of the circuitry. Another way of solving this problem is described in U.S. Pat. No. 3,965,330. The printer head disclosed therein utilizes heating resistors having a switching characteristic to eliminate the shadow problem. When a certain threshold voltage is reached in a particular resistor the power dissipated is greatly increased and the thermally sensitive paper is marked. The current travelling through unselected resistors is not great enough to cause the threshold voltage to be exceeded. Thus, power dissipation is not great enough in these resistors to mark the paper. An undesirable feature is that the fabrication of a matrix using resistors having switching characteristics is more difficult and more expensive than fabrication of a matrix using conventional resistance devices.